Gas turbine engines and other turbomachines have rows of wheel mounted blades which rotate within a generally cylindrical case or shroud. Such engines are generally driven by directing high temperature gas therethrough to cause the blades to rotate relative to the shroud. The gas is generally corrosive in nature due to the chemical makeup thereof. The blades may be coated with a thin protective coating to protect them from the corrosive action of the gas. The purpose of the shroud is to prevent gas from bypassing the blades. Without the shroud, the gas could flow outwardly of the radially outer end, or tip, of the blade. To minimize the amount of gas escaping between the tip of a rotor blade and the shroud, the operating clearance between the tip of the rotor blade and the shroud should be as small as is practical. Generally the length of the blade is selected such that the radially outer end, or tip, of the blade is disposed close enough to the inner surface of the shroud so as to form a seal therebetween. One of the problems encountered with such engines is that rubbing contact inevitably occurs between the blade tip and the shroud. The main cause of such rubbing is difference in thermal expansion and/or contraction between the turbine and the shroud. The immediate problem caused by the rubbing is that the blade and the shroud wear away resulting in eventual loss of efficiency. The far reaching effect is that the protective coating at the tip of the blade is worn away exposing more of the base metal to the corrosive gases and more rapid deterioration of the blade tip will occur.
Some users may want to burn low grade ashforming fuels which contain high amounts of corrosive elements. Price, availability, and flexibility of fuel supply requires the consideration of crude and residual oil for future industrial gas turbine installations. Other fuels such as low Btu gas from enhanced oil recovery, fire flood operations or other industrial processes are available. Marine operations, for example, occasionally inadvertently contaminate fuel with sea water. These criteria have led turbine manufacturers to develop technologies for successful gas turbine operation on such low grade or "dirty" fuels.
One of the major concerns in using lower grade fuels is the corrosion caused by various mineral elements contained in these fuels. Corrosion rates rapidly increase at metal temperatures above 650.degree. C. The temperature at the exposed turbine tip exceeds this temperature. Thus, actions must be taken to overcome the corrosion problems which erode the outer portion of turbine blades and the inwardly facing surface of shrouds affixed about the turbine blades.
The use of a coating of material on the radially outer edges of a blade tip has been suggested as a way of protecting the tip from the gases. However, due to the physical characteristics of present day coatings such coatings normally have a thickness of between 5 and 30 mils and will eventually be worn away. Since each such rub wears away some of the material, the radially thicker the coating, the more rubs it will withstand before it is completely worn away. If the coating is not of sufficient thickness to withstand these rubs the base blade material is exposed to the corrosive gases and rapid corrosion will occur. However, there is a maximum usable thickness limitation of the coating due to the lack of structural rigidity of the coating compared to the relatively high structural rigidity of the remainder of the tip. That is, if the coating was too thick radially, relative to the radial length of the blade, one rub could cause the entire coating of material to break off. Another problem resulting from the wearing away of the blade tip results in a greater gap of space between the blade and the shroud. The effects of this problem will be explained later. When the coating is provided on a superalloy turbine blade tip, the method of application must be metallurgically compatible with the superalloy substrate so that the properties of the substrate are not degraded. Such considerations place restraints on the kinds of coatings and processing techniques which are useful in the fabrication of such coatings.
Furthermore, structural integrity of the blades are absolutely essential due to the high centrifugal stresses and elevated temperatures to which the blades are exposed during high-speed rotation under normal operating conditions.
Examples of the above structures are described in the following patents: U.S. Pat. No. 4,390,320 issued to James E. Eiswerth on June 28, 1983, U.S. Pat. No. 4,689,242 issued to Roscoe A. Pike on Aug. 25, 1987, U.S. Pat. No. 4,589,823 issued to William K. Koffel on May 20, 1986, and U.S. Pat. No. 4,610,698 issued to Harry E. Eaton, et al on Sept. 9, 1986.
Other attempts to resist corrosion of the blade tip have resulted in various combinations of blade tips. For example, U.S. Pat. No. 4,232,995 issued to Kenneth W. Stalker, et al an Nov. 11, 1980, discloses an inner and outer tip portion bonded to a metallic projection body and the inner tip portion respectively. The inner tip is diffusion bonded to the body.
An example of rebuilding a portion of a blade disclosed in U.S. Pat. No. 3,574,924 issued to Gordon L. Dibble on Apr. 13, 1971. In that rebuilding process the damaged area of the blade is trimmed off and replaced with a precisely correspondingly sized replacement portion. The replacement portion is diffusion bonded in a mold to exactly duplicate the contour of the original blade. Another method for repairing blade tips is disclosed in U.S. Pat. No. 4,214,355 issued to John W. Zelahy on July 29, 1980. A first member is bonded to the side walls of a hollow body and a second member is bonded to the first member. Both members are diffusion bonded to the side walls and each other.
The coating patents listed above fail to provide a satisfactory tip on the blade which will withstand a multitude of rubs. The physical characteristics of todays coatings prevent the functional use of a thickness which will withstand a multitude of rubs which occurs with todays engines. The internal bond of todays coatings will allow the material to shear or break off from the blade body. Thus, heretofore a coating having a thickness greater than between 5 and 30 mils is not workable.
In todays applications attempts to increase turbine blade and shroud assembly resistance to high temperature, oxidation, corrosion and sulfidation have failed to consider that rubs are inevitable and that they must compensate for the affect that they have on the blade tip and shroud assembly.
The continued requirement by industry for use of dirty fuels has caused attempts to minimize the affects of these fuels on the turbine components. For example, the coating patents and the replacement portion or new portion patents listed above employ a diffusion bonding technique which uses heat and pressure to achieve the atomic bond therebetween. Furthermore, the disclosures in such above-mentioned patents do not consider the length required of a tip portion to insure a quantity of material resistant to oxidation and sulfidation that will remain after a multitude of rubs have occurred.
The greater the gap or spacing between the blades and the shroud the lower the efficiency of the engine. Thus, it is essential to provide as small a gap as is practical to insure maximum efficiency of the engine. In the art as shown above, as the blades wear the gap becomes greater and the efficiency is decreased. Therefore, it is desirable to prevent wear of the blades.
The present invention is directed to overcome one or more of the problems as set forth above.